The ability of natural products and other compounds to act as proteasome inhibitors has attracted significant interest because of the wide range of cellular substrates and processes controlled or affected by the ubiquitin-proteasome pathway. For example, the oscillation of cyclins (cell cycle proteins required for the orderly progression through the cell cycle) has been found to be due to the regulated degradation mediated by the ubiquitin-proteasome pathway, and inhibition of this pathway is believed to result in the blockage of cell cycle progression. Additionally, the transcription factor NF-kB is another regulatory protein involved in a variety of cellular processes, including immune and inflammatory responses, apoptosis, and cellular proliferation, whose mode of action is controlled by the ubiquitin-proteasome pathway. Furthermore, it has also been shown that the ubiquitin-proteasome pathway is involved in retrovirus assembly and thus may be a useful target for the development of anti-HIV drugs. For a general discussion of the ubiquitin-proteasome pathway and proteasome inhibitors see, Myung et al. “The Ubiquitin-Proteasome Pathway and Proteasome Inhibitors” Medicinal Research Reviews 2001, 21, 245-273.